The present invention relates to improved stator nozzles and fluid flow paths for turbine engines and power plants, particularly turbines used in Rankine cycle power systems. Small scale Rankine cycle power systems typically use high pressure ratio single-stage turbines. The high pressure ratios result in supersonic flow velocities at some stage in the turbine.
Axial flow turbines for these applications often have supersonic relative flow in both the stator nozzles and rotor blading. The efficiency of these small turbines is very sensitive to the contour of flow passages, and is adversely affected by manufacturing tolerances and finishing of the wall surfaces. As a result, the nozzle passages are often of axisymmetric configuration. These passages can be satisfactorily machined by a boring operation which allows close control of contour tolerances and good surface finishing. Circular nozzle flow paths also present a lower surface area than, for example, a rectangular nozzle of the same flow area, and boundry layer flow losses are lower.
Radial inflow turbines also have been used for small scale high pressure ratio applications. Due to the radius change of the mean flow path through the rotor, the radial inflow turbine can accommodate relatively high pressure ratios without the necessity of supersonic relative rotor inlet velocities. A supersonic nozzle exit velocity is required, however.
The nozzles of radial inflow turbines direct the flow in the radial and tangential planes without any axial component. It is not possible with such turbines to bore circular cross section converging-diverging nozzles because the circumferential curvature of the exit plane does not allow sufficient tool clearance for machining the diverging portion from the discharge side. Radial inflow supersonic nozzle rings must therefore be constructed in two pieces with one shroud removable to permit machining of the passages from the side. Such nozzle passageways are usually rectangular in cross section and are subject to more difficulty than with axisymmetric bored passageways in the control of contour tolerances and the finishing of the wall surfaces.
From an efficiency standpoint, radial inflow turbines have a higher potential efficiency than axial flow turbines. At the same work level, the radial inflow turbines have a lower absolute exit velocity, a lower exhaust energy level, and thus a higher overall (total to static) potential efficiency.
The present invention has as its overall object to improve turbine engines and more particularly to improve turbines for use in high pressure ratio single-stage Rankine engines. Another object is to provide improved stator nozzles and fluid flow paths for such turbine engines. A further object is to provide a turbine which is an improvement over both axial flow turbines and radial flow turbines and which overcomes the disadvantages of each one.
Still further objects and advantages of the invention will become apparent upon consideration of the present disclosure and the accompanying drawings and claims.